i think its a fear of not being able to love again, or a fear of getting hurt again...when we are in deep relationships, we learn alot about ourselves and who/what we are attracted to. this enables us to make better chioces the next time around. its about finding a more "suitable" person for "us"....more so than a "better" or just as "good" a person as we once had.
if you were to try and search for an exact or close replica- youre in for a very long haul, most likely leading to nowhere.
when we come out of exisiting realtionships, we are not the same as before. this is providing the fact you are not a CO-DEPENDANT and keep seeking poor helpless/hapless mates...we all know some people in our lives, who seem to pick the same loosers time and time again...but the problem stems from themselves, not who they are picking out.